Production of nickelized shaped articles

ABSTRACT

Nickelized shaped articles are produced by electrodeposition of nickel from aqueous acidic baths containing as essential constituents one or more nickel salts, one or more inorganic acids and at least two brighteners by using as brighteners a mixture of 
     A) from 2 to 98% by weight of one or more cyclic N-allylammonium or N-vinylammonium compounds I ##STR1##  where the nitrogen atom is part of a pyridine, quinoline or isoquinoline ring system which may additionally carry one or two C 1  - to C 4  -alkyl substituents or halogen atoms, 
     R 1 , R 3  and R 4  are each hydrogen or C 1  - to C 4  -alkyl, 
     R 2  is hydrogen or methyl, 
     m is from 0 to 4, 
     n is from 1 to 4, and 
     X.sup.⊖ is an n-valent inorganic or organic anion which promotes solubility in water, and 
     B) from 98 to 2% by weight of one or more acetylenically unsaturated compounds II 
     
         R.sup.4 --C.tbd.C--R.sup.5                                 (II) 
    
      where R 4  and R 5  are identical or different and each is C 1  - to C 4  -alkyl substituted by hydroxyl, sulfo, amino, C 1  - to C 4  -alkylamino or di(C 1  - to C 4  -alkyl)amino, although hydroxyl groups may have been reacted with from 1 to 10 mol of a C 2  - to C 4  -alkylene oxide or a mixture of such alkylene oxides and one of the radicals R 4  and R 5  may also be hydrogen or C 1  - to C 4  -alkyl.

The present invention relates to an improved process for producingnickelized shaped articles by electrodeposition of nickel from aqueousacidic baths containing as essential constituents one or more nickelsalts, one or more inorganic acids and at least two brighteners, and toa brightener mixture usable for this purpose.

It is known that acidic nickel electrolytes must contain small amountsof organic substances if the electronickelization is to produce abright, ductile and surface-planar deposition of the metal. Suchbrighteners, which in general are divided into primary and secondarybrighteners, are customarily used in the form of combinations comprisinga plurality of these agents in order to enhance the effect.

Praktische Galvanotechnik, Eugen G. Lenze Verlag, Saulgau, 4th edition1984, pages 268 to 271 (1) describes customary brighteners for nickelelectrolytes. Although the compounds are classified as primary orsecondary brighteners or planarizers, it is admitted at the same timethat clear-cut classification is not always possible. The brighteningcompounds mentioned are:

sulfonimides, eg. benzoic sulfimide

sulfonamides

benzenesulfonic acids, eg. mono-, di- and tri-benzenesulfonic acid

naphthalenesulfonic acids, eg. mono-, di- and tri-naphthalenesulfonicacid

alkylsulfonic acids

sulfinic acid

arylsulfone sulfonates

aliphatic compounds with ethylene and/or acetylene bonds, eg. butynediol

single- and multiring nitrogen-containing heterocycles with or withoutfurther hetero atoms such as

sulfur or selenium

coumarin

amines and quaternary ammonium compounds as planarizing agents

saccharin.

DE-B-1 191 652 (2) describes single- or multiring heterocyclic nitrogenbases of the aromatic type in quaternized form such as pyridinium salts,eg. 2-pyridinium-1-sulfatoethane, as planarizers, ie. brightners, foracidic nickel-plating baths. These agents are used together withcustomary basic brighteners such as benzene-m-disulfonic acid,diaryldisulfimides or sulfonamides.

U.S. Pat. No. 2,647,866 (3) describes mixtures of N-substitutedpyridinium compounds, eg. N-allylpyridinium or N-methallylpyridiniumhalides, with certain sulfur-containing compounds, eg. allylsulfonicacid, as highly effective brighteners in the electrodeposition ofnickel. A use concentration of from 4 to 12 g/l is recommended forallylsulfonic acid.

However, the prior art agents generally need to be used in relativelyhigh concentrations in the nickel electrolyte baths used; what is more,the brightness and planarization achievable with these agents are stillin need of improvement.

It is an object of the present invention to remedy the aforementioneddefects of the prior art.

We have found that this object is achieved by a process for producingnickelized shaped articles by electrodeposition of nickel from aqueousacidic baths containing as essential constituents one or more nickelsalts, one or more inorganic acids and at least two brighteners, whichcomprises using as brighteners a mixture of

A) from 2 to 98% by weight of one or more cyclic N-allylammonium orN-vinylammonium compounds of the general formula I ##STR2## where thenitrogen atom is part of a pyridine, quinoline or isoquinoline ringsystem which may additionally carry one or two C₁ - to C₄ -alkylsubstituents or halogen atoms,

R¹, R³ and R⁴ are each hydrogen or C₁ - to C₄ -alkyl,

R² is hydrogen or methyl,

m is from 0 to 4,

n is from 1 to 4, and

X.sup.⊖ is an n-valent inorganic or organic anion which promotessolubility in water, and

B) from 98 to 2% by weight of one or more acetylenically unsaturatedcompounds of the general formula II

    R.sup.4 --C.tbd.C--R.sup.5                                 (II)

where R⁴ and R⁵ are identical or different and each is C₁ - to C₄ -alkylsubstituted by hydroxyl, sulfo, amino, C₁ - to C₄ -alkylamino or di(C₁ -to C₄ -alkyl)-amino, although hydroxyl groups may have been reacted withfrom 1 to 10 mol of a C₂ - to C₄ -alkylene oxide or a mixture of suchalkylene oxides and one of the radicals R⁴ and R⁵ may also be hydrogenor C₁ -to C₄ -alkyl.

Suitable C₁ - to C₄ -alkyl radicals are in general n-propyl, isopropyl,n-butyl, isobutyl, sec-butyl, tertbutyl and in particular methyl andethyl.

Examples of alkyl- or halogen-substituted heterocycli ring systems for Iare:

2-, 3- or 4-methylpyridinium,

2-, 3- or 4-chloropyridinium,

2-, 3- or 4-bromopyridinium,

2-, 3- or 4-ethylpyridinium,

3- or 4-n-propylpyridinium,

3- or 4-isopropylpyridinium,

3- or 4-n-butylpyridinium,

4-tert-butylpyridinium,

2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dimethylpyridinium,

3-methyl-4-isopropylpyridinium,

4-tert-butyl-3-methylpyridinium,

2-, 3-, 4-, 5-, 6-, 7- or 8-methylquinolinium,

2- or 6-bromoquinolinium,

2,4-dichloroquinolinium,

2,4-, 2,8- or 5,8-dimethylquinolinium and

1-, 3-, 4-, 5-, 6-, 7- or 8-methylisoquinolinium.

The term halogen atom herein encompasses fluorine, iodine and inparticular bromine and chlorine.

Preference is given to unsubstituted pyridinium.

R¹, R³ and R⁴ are preferably each hydrogen.

The variable m is preferably 1.

Suitable n-valent anions X are the customary, normallywater-solubilizing inorganic or organic anions, in particular chloride,bromide, fluoride, sulfate, hydrogensulfate, methanesulfonate,trifluoromethanesulfonate, 2-hydroxyethanesulfonate, p-toluenesulfonate,nitrate, tetrafluoroborate, perchlorate,1-hydroxyethane-1,1-diphosphonate, dihydrogenphosphate, phosphate,formate, acetate, oxalate and tartrate.

Of these, anions with one or two charges (n=1 or 2) are preferred, inparticular fluoride, sulfate, methanesulfonate, nitrate andtetrafluoroborate but especially chloride and bromide.

In a preferred embodiment, component A comprises cyclic N-allylammoniumcompounds of the general formula ##STR3## where R², n and X.sup.⊖ areeach as defined above.

The cyclic N-allylammonium or N-vinylammonium compounds I and Ia arebest prepared in a conventional manner by reacting the correspondingprecursor of the general formula ##STR4## where Z is a nucleofugicleaving group, preferably chlorine or bromine, with a heterocycle of thegeneral formula IV ##STR5## and if desired subsequently exchanging theanion Z.sup.⊖ for X.sup.⊖.

The reaction of components III and IV is advantageously carried out inan inert organic solvent such as toluene, xylene, petroleum ether,naphtha, cyclohexane, acetone, tetrahydrofuran, dioxane, methanol,ethanol, isopropanol, ethyl acetate or methyl benzoate or in a mixturethereof. However, the reaction can also be carried out in water or in asingle-phase or two-phase mixture of water and one or more organicsolvents. In the case of two-phase mixtures it is possible to use acustomary phase transfer catalyst. The reaction is in general carriedout at from 40° to 130° C., in particular at from 60° to 110° C., underatmospheric pressure.

Component B comprises acetylenically unsaturated compounds II whichcarry at least one polar functional group, preferably in the α-positionof the C₁ - to C₄ -alkyl radicals. Nonfunctionalized acetylenicallyunsaturated hydrocarbons are unsuitable, mainly because of their lowsolubility in aqueous media and because of their high volatility.

If hydroxyl-containing compounds II are used, they may have beenalkoxylated with a C₂ - to C₄ -alkylene oxide such as ethylene oxide,propylene oxide or butylene oxide or a mixture of such alkylene oxides.If alkoxylated, the degree of alkoxylation is preferably from 1 to 3, inparticular 1.

Sulfo-containing compounds II can be used in the form of the free acidor preferably in the form of their alkali metal, e.g. sodium orpotassium, salts or ammonium salts.

In a preferred embodiment, component B comprises acetylenicallyunsaturated compounds II where R⁴ and R⁵ are identical or different andeach is hydroxyl-, sulfo-, dimethylamino- or diethylamino-substitutedmethyl or ethyl, although hydroxyl groups may have been reacted withfrom 1 to 3 mol of a C₂ - to C₄ -alkylene oxide or a mixture of suchalkylene oxides and one of the radicals R⁴ and R⁵ may also be hydrogen,methyl or ethyl.

Examples of compounds II are:

prop-2-yn-1-ol,

but-2-yne-1,4-diol,

hex-3-yne-2,5-diol,

prop-2-yn-1-ol ethoxylate,

prop-2-yn-1-ol propoxylate,

but-2-yne-1,4-diol monoethoxylate,

but-2-yne-1,4-diol diethoxylate,

but-2-yne-1,4-diol monopropoxylate,

but-2-yne-1,4-diol dipropoxylate,

prop-2-yne-1-sulfonic acid,

1-dimethylaminoprop-2-yne,

1-diethylaminoprop-2-yne,

1-diethylaminopent-2-yn-4-ol and

1-amino-1,1-dimethylprop-2-yne.

The terms ethoxylate, monoethoxylate, diethoxylate, propoxylate,monopropoxylate and dipropoxylate in each case denote the reactionproducts with 1 mol of alkylene oxide per hydroxyl group on thecompounds II.

The mixture of the cyclic N-allylammonium or N-vinylammonium compounds Iand the acetylenically unsaturated compounds II preferably comprisesfrom 10 to 90% by weight, in particular from 25 to 75% by weight, ofcomponent A and from 90 to 10% by weight, in particular from 75 to 25%by weight, of component B. Very particular preference is given to amixture of from 28 to 50% by weight of A and from 72 to 50% by weight ofB.

In a preferred embodiment, component A is a mixture of one or morecyclic N-allylammonium or N-vinylammonium compounds I and component B isat least three, in particular from three to seven, acetylenicallyunsaturated compounds II.

Furthermore, the mixtures of the components A and B can be used togetherwith other customary brighteners, for example with saccharin,allylsulfonic acid or alkali metal or ammonium salts thereof,vinylsulfonic acid or alkali metal or ammonium salts thereof,bisbenzenesulfonylimide or commercial brighteners based on a thiocompound.

The aqueous acidic nickel electrolyte baths used contain one or usuallymore than one nickel salt, for example nickel sulfate and nickelchloride, one or more inorganic acids, preferably boric acid andsulfuric acid, as brighteners the mixture of components A and B alone orcombined with further, customary brighteners and optionally furthercustomary auxiliaries and additives in the concentrations customarytherefor, for example wetting agents and pore inhibitors. Customaryaqueous acidic nickel electrolytes ("Watts elektrolytes") have thefollowing basic composition:

200-350 g/l of NiSO₄. 7 H₂ O

30-150 g/l of NiCl₂. 6 H₂ O

30-50 g/l of H₃ BO₃.

The electrolyte bath pH is customarily within the range from 3 to 6,preferably within the range from 4 to 5. This pH is conveniently setwith a strong mineral acid, preferably sulfuric acid.

The mixture of components A and B is present in the electrolyte baths inlow concentrations, in general within the range from 0.05 to 1.0 g/l,preferably within the range from 0.1 to 0.5 g/l. The concentrations offurther, customary brighteners are in each case normally within therange from 0.1 to 10 g/l, in particular from 0.1 to 2.0 g/l.

The nickel electrolyte baths described above can be used to electroplatein particular nickel coatings onto shaped articles made of steel, butalso onto shaped articles made of other materials, for example brass,which have been pretreated as usual. The electroplating is in generalcarried out at from 30° to 80° C., preferably at from 40° to 60° C.

The present invention also provides a brightener mixture for aqueousacidic baths for the electrodeposition of nickel, comprising a mixtureof

A) from 2 to 98% by weight of one or more cyclic N-allylamonium orN-vinylammonium compounds of the general formula I ##STR6## where thenitrogen atom is part of a pyridine, quinoline or isoquinoline ringsystem which may additionally carry one or two C₁ - to C₄ -alkylsubstituents or halogen atoms,

R¹, R³ and R⁴ are each hydrogen or C₁ - to C₄ -alkyl,

R² is hydrogen or methyl,

m is from 0 to 4,

n is from 1 to 4, and

X.sup.⊖ is an n-valent inorganic or organic anion which promotessolubility in water, and

B) from 98 to 2% by weight of one or more acetylenically unsaturatedcompounds of the general formula II

    R.sup.4 --C.tbd.C--R.sup.5                                 (II)

where R⁴ and R⁵ are identical or different and each is C₁ - to C₄ -alkylsubstituted by hydroxyl, sulfo, amino, C₁ - to C₄ -alkylamino or di(C₁ -to C₄ -alkyl)amino, although hydroxyl groups may have been reacted withfrom 1 to 10 mol of a C₂ - to C₄ -alkylene oxide or a mixture of suchalkylene oxides and one of the radicals R⁴ and R⁵ may also be hydrogenor C₁ -to C₄ -alkyl.

The brightener mixture of the invention is notable for extremelypowerful brightening. In general it produces better brightness andbetter planarization than the customary brighteners at a distinctlylower concentration in the nickel electrolyte bath.

USE EXAMPLES

The mixtures listed in the Table below as Examples 1 to 11 were used asbrighteners in weakly acidic electroplating baths for the deposition ofnickel.

The aqueous nickel electrolyte used had the following composition:

300 g/l of NiSO₄.7 H₂ O

60 g/l of NiCl₂.6 H₂ O

45 g/l of H₃ BO₃

0.5 g/l of a fatty alcohol derivative of the formula C₁₂ H₂₅ /C₁₄ H₂₉--O--(CH₂ CH₂ O)₂ --SO₃ Na as wetting agent.

The respective brightener concentrations are indicated below in theTable.

The pH of the electrolyte was set to 4.2 with sulfuric acid.

Brass panels were used. Prior to being coated with nickel they had beencathodically degreased in a conventional manner in an alkalineelectrolyte. They were nickelized in a 250 ml Hull cell at 55° C. with acurrent strength of 2A over a period of 10 minutes. The panels were thenrinsed with water and dried with compressed air.

The Table below shows the results of these experiments. It can be seenthat the brightener mixture of the invention produced better brightnessand planarization than the prior art brighteners, in some instances at adistinctly lower concentration in the nickel electrolyte bath.

                  TABLE                                                           ______________________________________                                        Test results of electronickelization                                                                Concentra-                                                                              Bright-                                                                             Planari-                                Ex.                   tion      ness  zation                                  No.  Brightener       [g/l]     [rating]                                                                            [rating]                                ______________________________________                                        1    Component A:     0.1       5     2                                            N-allylpyridinium chloride                                                    Component B:     0.15                                                         but-2-yne-1,4-diol                                                            further brighteners:                                                                           2                                                            saccharin                                                                2    Component A:     0.1       5     2                                            N-allylpyridinium chloride                                                    Component B:     0.25                                                         but-2-yne-1,4-diol mono-                                                      propoxylate                                                                   further brighteners:                                                                           6                                                            sodium allylsulfonate                                                    3    Component A:     0.1       5     2                                            N-allylpyridinium chloride                                                    Component B:                                                                  but-2-yne-1,4-diol                                                                             0.1                                                          prop-2-yn-1-ol   0.02                                                         prop-2-yn-1-ol ethoxylate                                                                      0.04                                                         further brighteners:                                                          saccharin        2                                                            sodium allylsulfonate                                                                          4                                                            bisbenzenesulfonylimide                                                                        0.5                                                     4    Component A:     0.1       5     2                                            N-allylpyridinium chloride                                                    Component B:                                                                  but-2-yne-1,4-diol mono-                                                                       0.08                                                         propoxylate                                                                   prop-2-yn-1-ol ethoxylate                                                                      0.015                                                        but-2-yne-1,4-diol                                                                             0.015                                                        diethoxylate                                                                  further brighteners:                                                          saccharin        2                                                            sodium allylsulfonate                                                                          4                                                       5    Component A:     0.1       5     2                                            N-allylpyridinium chloride                                                    Component B:                                                                  prop-2-yn-1-ol   0.01                                                         but-2-yne-1,4-diol mono-                                                                       0.08                                                         propoxylate                                                                   prop-2-yne-1-sulfonic acid,                                                                    0.05                                                         sodium salt                                                                   prop-2-yn-1-ol propoxylate                                                                     0.02                                                         further brighteners:                                                          saccharin        2                                                            sodium allylsulfonate                                                                          4                                                       6    Component A:     0.1       5     2                                            N-allylpyridinium chloride                                                    Component B:                                                                  prop-2-yn-1-ol ethoxylate                                                                      0.015                                                        but-2-yne-1-4-diol mono-                                                                       0.015                                                        ethoxylate                                                                    1-diethylamino-prop-2-yne                                                                      0.01                                                         prop-2-yne-1-sulfonic acid,                                                                    0.1                                                          sodium salt                                                                   prop-2-yn-1-ol propoxylate                                                                     0.01                                                         further brighteners:                                                          saccharin        2                                                            sodium vinylsulfonate                                                                          1                                                            bisbenzenesulfonylimide                                                                        0.5                                                          commercial brightener                                                         based on a thio compound                                                 7    Component A:     0.1       5     2                                            N-allylpyridinium chloride                                                    Component B:                                                                  prop-2-yn-1-ol   0.015                                                        prop-2-yn-1-ol ethoxylate                                                                      0.02                                                         but-2-yne-1,4-diol mono-                                                                       0.05                                                         propoxylate                                                                   but-2-yne-1,4-diol mono-                                                                       0.01                                                         ethylate                                                                      1-diethylamino-prop-2-yne                                                                      0.01                                                         prop-2-yne-1-sulfonic acid,                                                                    0.03                                                         sodium salt                                                                   prop-2-yn-1-ol propoxylate                                                                     0.01                                                         further brighteners:                                                          saccharin        2                                                            sodium vinylsulfonate                                                                          0.8                                                          bisbenzenesulfonylimide                                                                        0.05                                                         commercial brightener                                                                          0.1                                                          based on a thio compound                                                 8    Component A:     0.1       5     2                                            N-methallylpyridinium                                                         chloride                                                                      Component B:                                                                  but-2-yne-1,4-diol mono-                                                                       0.08                                                         propoxylate                                                                   but-2-yne-1,4-diol mono-                                                                       0.015                                                        ethoxylate                                                                    prop-2-yn-1-ol ethoxylate                                                                      0.015                                                        further brighteners:                                                          saccharin        2                                                            sodium allylsulfonate                                                                          4                                                       9    Component A:     0.1       5     2                                            N-methallylpyridinium                                                         chloride                                                                      Component B:                                                                  prop-2-yn-1-ol   0.01                                                         but-2-yne-1,4-diol mono-                                                                       0.08                                                         propoxylate                                                                   prop-2-yne-1-sulfonic acid,                                                                    0.05                                                         sodium salt                                                                   prop-2-yn-1-ol ethoxylate                                                                      0.02                                                         further brighteners:                                                          saccharin        2                                                            sodium allylsulfonate                                                                          4                                                       10   Component A:     0.1       5     2                                            N-allylpyridinium                                                             chloride                                                                      Component B:     0.25                                                         prop-2-yne-1-sulfonic acid,                                                   sodium salt                                                              11   Component A:     0.1       5     2                                            N-methallylpyridinium                                                         chloride                                                                      Component B:     0.25                                                         prop-2-yne-1-sulfonic acid,                                                   sodium salt                                                              For comparison:                                                               A    N-allylpyridinium chloride                                                                     0.1        4    1                                            sodium allylsulfonate                                                                          6                                                       B    N-methallylpyridinium                                                                          0.1       4     1                                            chloride                                                                      Sodium allylsulfonate                                                                          6                                                       ______________________________________                                    

Comparative mixtures A and B are known from reference (3).

Rating Scheme for Brightness

5=excellent (perfect specular gloss)

4=good (virtually specular gloss)

3=moderate

2=poor

1=no brightness

Rating scheme for planarization

2=high

1=medium

0=moderate

We claim:
 1. A process for producing nickelized shaped articles byelectrodeposition of nickel from aqueous acidic baths containing asessential constituents one or more nickel salts, one or more inorganicacids and at least two brighteners, which comprises using as brightenersa mixture ofA) from 2 to 98% by weight of one or more cyclicN-allylammonium or N-vinylammonium compounds of the general formula I##STR7## where the nitrogen atom is part of a pyridine, quinoline orisoquinoline ring system which may additionally carry one or two C₁ - toC₄ -alkyl substituents or halogen atoms,R¹, R³ and R⁴ are each hydrogenor C₁ - to C₄ -alkyl, R² is hydrogen or methyl, m is from 0 to 4, n isfrom 1 to 4, and X.sup.⊖ is an n-valent inorganic or organic anion whichpromotes solubility in water, and B) from 98 to 2% by weight of one ormore acetylenically unsaturated compounds of the general formula II

    R.sup.4 --C.tbd.C--R.sup.5                                 (II)

where R⁴ and R⁵ are identical or different and each is C₁ - to C₄ -alkylsubstituted by hydroxyl, sulfo, amino, C₁ - to C₄ -alkylamino or di(C₁ -to C₄ -alkyl)amino, although hydroxyl groups may have been reacted withfrom 1 to 10 mol of a C₂ - to C₄ -alkylene oxide or a mixture of suchalkylene oxides and one of the radicals R⁴ and R⁵ may also be hydrogenor C₁ -to C₄ -alkyl.
 2. A process for producing nickelized shapedarticles as claimed in claim 1, wherein component A comprises cyclicN-allylammonium compounds of the general formula Ia ##STR8## where R², nand X.sup.⊖ are each as defined above.
 3. A process for producingnickelized shaped articles as claimed in claim 1, wherein component Bcomprises acetylenically unsaturated compounds II where R⁴ and R⁵ areidentical or different and each is hydroxyl-, sulfo-, dimethylamino- ordiethylamino-substituted methyl or ethyl, although hydroxyl groups mayhave been reacted with from 1 to 3 mol of a C₂ - to C₄ -alkylene oxideor a mixture of such alkylene oxides and one of the radicals R⁴ and R⁵may also be hydrogen, methyl or ethyl.
 4. A process for producingnickelized shaped articles as claimed in claim 1, wherein component A isa mixture of one or more cyclic N-allyammonium or N-vinylammoniumcompounds I and component B is at least three acetylenically unsaturatedcompounds II.
 5. A brightener mixture for aqueous acidicelectronickelizing baths, comprising a mixture ofA) from 2 to 98% byweight of one or more cyclic N-allylammonium or N-vinylammoniumcompounds of the general formula I ##STR9## where the nitrogen atom ispart of a pyridine, quinoline or isoquinoline ring system which mayadditionally carry one or two C₁ - to C₄ -alkyl substituents or halogenatoms,R¹, R³ and R⁴ are each hydrogen or C₁ - to C₄ -alkyl, R² ishydrogen or methyl, m is from 0 to 4, n is from 1 to 4, and X.sup.⊖ isan n-valent inorganic or organic anion which promotes solubility inwater, and B) from 98 to 2% by weight of one or more acetylenicallyunsaturated compounds of the general formula II

    R.sup.4 --C.tbd.C--R.sup.5                                 (II)

where R⁴ and R⁵ are identical or different and each is C₁ - to C₄ -alkylsubstituted by hydroxyl, sulfo, amino, C₁ - to C₄ -alkylamino or di(C₁ -to C₄ -alkyl)amino, although hydroxyl groups may have been reacted withfrom 1 to 10 mol of a C₂ - to C₄ -alkylene oxide or a mixture of suchalkylene oxides and one of the radicals R⁴ and R⁵ may also be hydrogenor C₁ -to C₄ -alkyl.